Imaging devices, including charge coupled devices (CCD) and complementary metal oxide semiconductor (CMOS) sensors, and others, have commonly been used in photo-imaging applications. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, for example, a photogate, photoconductor or a photodiode for accumulating photo-generated charge in the specified portion of the substrate. Each pixel cell has a charge storage region, formed on or in the substrate, which is connected to the gate of an output transistor that is part of a readout circuit. The charge storage region may be constructed as a floating diffusion region. In some imager circuits, each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference.
In a CMOS imager, the active elements of a pixel cell perform the functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region; (5) selection of a pixel for readout; and (6) output and amplification of signals representing pixel reset level and pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,140,630, 6,177,333, 6,204,524, 6,310,366, 6,326,652, and 6,333,205, assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
CMOS, CCD and other solid state imagers may use an optical filter to transmit image light to a solid state sensor having an array of pixels. Optical filters employing grating elements can be used as a global light wavelength filter over the top of an imager to filter light across an entire array, for example, as an infra-red (IR) light block. In addition, optical filters having grating elements are now being fabricated as nano-structure-based optics or nano-optics, which are a class of optical devices that allow optical devices to be realized that are thin, offer high performance and are highly reliable. See Kostal et al., MEMS Meets Nano-Optics, FIBER OPTIC TECHNOLOGY, November 2005, pp. 8-13; see also NanoOpto Introduces New Nano-optic Bandpass Filter Designed for High Volume, High Performance Digital Imaging Applications, Press Release, NanoOpto Corporation, Jan. 31, 2005.
One drawback with an optical filter is that as the angle of incidence of incoming light gets sharper, i.e., begins angling away from the light path which is normal to the grating, the filter fails to block undesired wavelengths and/or fails to allow the desired light wavelengths to pass through. As a result, the optical filter is less efficient in its filtering characteristics, which can result in image color shading and artifact formation.
Accordingly, there is a need and desire for an apparatus and method for improving an optical filter employing grating elements for use in imaging devices.